Fiber-like Micelles via the Crystallization-Driven Solution Self-Assembly of Poly(3-hexylthiophene)-block-Poly(methyl methacrylate) Copolymers

The solution self-assembly of block copolymers with a pi-conjugated, crystalline, core-forming block represents a facile strategy toward the preparation of semiconducting nanowires with potential for high-tech applications. In this study, two asymmetric block copolymers based on regioregular poly(3-hexylthiophene) (P3HT) and poly(methyl methacrylate) (PMMA), namely P3HT(40)-b-PMMA(520) (6a) and P3HT(40)-b-PMMA(1100) (613) (block ratios = 1:13 and 1:27.5, respectively) were prepared via atom transfer radical polymerization (ATRP) from a P3HT macroinitiator. The solution self-assembly of the P3HT-b-PMMA block copolymers was subsequently studied under a variety of experimental conditions. Short, fiber-like micelles resulted when THF (common solvent for P3HT and PMMA) solutions of the block copolymer were dialyzed against ethyl acetate and n-butyl acetate (selective solvents for PMMA). The electronic properties of the fiber-like micelles obtained coupled with wide-angle X-ray scattering studies confirmed that the cores of the aggregates were crystalline and suggested that growth occurs via a crystallization-driven pathway. The average lengths of fiber-like micelles were shown to increase relative to those obtained from dialysis versus the PMMA selective solvent, when THF was slowly evaporated from mixtures containing n-butyl acetate and P3HT-b-PMMA unimers, thereby limiting the rate of P3HT aggregation. Furthermore, the formation of only relatively short (mainly under 200 nm, always <1 mu m) fiber-like micelles in these studies, even when the ratio of THF/alkyl acetate was controlled carefully via dialysis or evaporation, indicated that homogeneous nucleation of P3HT-b-PMMA block copolymers is relatively facile. This behavior differs significantly from that detected for other block copolymers such as polyferrocenylsilane-based materials that undergo crystallization-driven self-assembly to form cylinders with lengths of up to 10 mu m under analogous conditions.